(1) Field of the Invention
The present invention relates to a heat-resistant paper container and a process for the preparation thereof. More particularly, the present invention relates to a heat-resistant paper container, the content of which can be heated and cooked by a microwave oven, an electric oven or an oven toaster, and a process for the preparation thereof.
(2) Description of the Prior Art
A tray-shaped paper container is widely used as a container in which a content such as a food is simply and easily packaged. With the recent spread of an oven, a microwave oven, an oven toaster and the like, development of a tray-shaped container which is sold in the state filled with a precooked or uncooked food and which is placed in a heating device as mentioned above to heat or cook the food for eating is desired.
As means meeting this desire, Japanese Patent Publication No. 41890/82 discloses a process for preparing a paper for a food container, which comprises forming a paper stock which is substantially neutral, impregnating the paper stock with an aqueous dispersion containing an inorganic filler, forming a starting paper from the stock, applying a heat-resistant coating on both the surfaces by bonding or pasting and coating a heat-resistant resin on the surface to be formed into an inner surface of the container. As the heat-resistant coating, there can be used not only an aluminum foil but also a nitro cellulose type lacquer and a resin coating of the epoxy, urethane or fluorine type. It is taught that on the inner surface side of the container, a heat-resistant resin of the silicone type or the like is applied as a releasing agent on the heat-resistant coating.
This container can be applied to the use where a starting material such as a sweet rice jelly or sponge cake is packaged and heat-treated at a temperature of about 200 to about 250.degree. C. However, in the case where heating or cooking is carried out by an oven toaster or the like, the temperature of the container is elevated to a level exceeding 300.degree. C., and the heat resistance of the above-mentioned paper container is still insufficient and the container cannot be applied to this high-temperature use.
In cellulose fibers of paper, carbonization and discoloration start at about 260.degree. C., and the fibers are completely carbonized at about 300.degree. C. Accordingly, when a paper container is applied to the use where the container is heated above 300.degree. C., the appearance of the container is blackened so that the container cannot be put into practical use, and the strength of the container per se is drastically reduced.
As means for hiding discoloration of a paper container, there should naturally be considered a method in which a coating layer comprising a hiding pigment and a resin binder is formed on the surface of a paper substrate. However, this coated paper is generally poor in the elongation necessary for molding and the press moldability to a tray is insufficient. If a pressing mold is heated to improve the moldability, the coating layer adheres to the mold and molding often becomes impossible. Even if the coating layer does not adhere to the pressing mold, the viscous resin component is gradually accumulated on the surface of the mold, resulting in reduction of the adaptability to the molding operation and occurrence of appearance defects of the formed container. This tendency may be moderated by applying a releasing agent to the mold surface but a drastic solution of the problem is not attained.
In addition to the above-mentioned problem concerning the molding, the conventional tray container composed of a coated paper involves a problem of generation of an unpleasant smell on heating in an oven, and the flavor of a packaged food or the like is degraded.